Since July 1970, 491 Hancock "SGP" bioprostheses have been implanted in 392 patients with 69 patients followed more than four years. Followup data include early and late mortality and causes, thromboembolic evidence, valve failures, six months postoperative hemodynamic data for atrioventricular and aortic implants, late (greater than 4.5 years) hemodynamic data for mitral valve patients, and complications following valve replacement with the "SGP" bioprosthesis. Durability of the bioprosthesis implanted more than 4.5 years is marginal. Long-term followup reveals an 8.7 percent valve failure after 4.5 years and late hemodynamic findings consistent with prosthetic stenosis or regurgitation in 4 of 10 patients. All other factors continue to be favorable regarding the Hancock "SGP" bioprosthesis.